CN102445769A - Optical modulator - Google Patents

Abstract

An object of the present invention is to obtain an optical modulator capable of sufficiently expanding a modulation frequency band. A waveguide (12) is arranged within the semiconductor chip (10). The power supply line (20) is connected to the input part (14a) of the traveling wave electrode (14) via the first wire (18). The terminal resistance (26) is connected to the output part (14b) of the traveling wave electrode (14) via the second wire (22) and the terminal line (24). The capacitance between the output part (14b) and the ground point is larger than the capacitance between the input part (14a) and the ground point.

Description

light modulator

technical field

The present invention relates to a traveling-wave optical modulator that operates at high speed, and in particular, to an optical modulator that can sufficiently expand a modulation frequency band.

Background technique

In the traveling wave optical modulator, when the light passes through the interference arm (arm), the microwave on which the modulation signal is superimposed also passes through the interference arm at approximately the same speed. At this time, the electric field of the microwave is applied to the interference arm, and the light is modulated. In the traveling wave optical modulator, compared with the lumped constant optical modulator, the modulation frequency band can be expanded without being limited by the capacitance of the waveguide.

Thus, in the traveling wave optical modulator, the waveguide is used as a microwave transmission line. In order to propagate the fundamental mode, the thickness of the core layer is preferably about 0.2 μm, and the width of the waveguide is about 2 μm. However, in this case, the impedance of the waveguide becomes as low as 35Ω, which causes a mismatch with the impedance (usually 50Ω) of the power supply line outside the chip. As a result, the light entering the optical modulator is reflected or attenuated, and the modulation frequency band is narrowed.

In order to increase the impedance against microwaves, it is necessary to thicken the depleted core layer and narrow the width of the waveguide. However, when the core layer is thick, the propagation mode of light becomes a higher-order mode instead of the fundamental mode, the extinction ratio deteriorates, the operating voltage increases, and the optical modulator does not operate. In addition, when the width of the high-mesa waveguide is narrow, the light does not propagate and the loss increases. In this way, one waveguide is used for both light and microwaves, but the optimum dimensions of the waveguides differ for both.

In addition, in a traveling wave Mach-Zehnder optical modulator using generally used lithium niobate (LiNbO ₃ ) as a material, since the dielectric constant of the material is low, the impedance of the waveguide can be set to 50Ω. In addition, in the lithium niobate modulator, in order to expand the modulation frequency band, for example, the scheme of reducing the output impedance by reducing the terminal resistance, or connecting a stub (stub) to the terminal resistance has been proposed (for example, refer to Patent Documents 1 to 2). 6).

[Patent Document 1]: Japanese Unexamined Patent Publication No. 2004-170931

[Patent Document 2]: Japanese Unexamined Patent Publication No. 2007-010942

[Patent Document 3]: Publication No. WO2005/096077

[Patent Document 4]: Japanese Patent Laying-Open No. 11-183858

[Patent Document 5]: Japanese Patent Application Laid-Open No. 07-221509

[Patent Document 6]: WO2010/001986 publication.

In a traveling-wave Mach-Zehnder optical modulator using a semiconductor as a material, since the material has a high dielectric constant, the impedance of the waveguide is 50Ω or less. Furthermore, the capacitance per unit length of the waveguide is large. Therefore, even if the termination resistance is reduced, the modulation frequency band cannot be sufficiently expanded. In addition, even if a stub is connected to the terminating resistor, the modulation frequency band cannot be sufficiently expanded.

Contents of the invention

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to obtain an optical modulator capable of sufficiently expanding a modulation frequency band.

The optical modulator of the present invention is characterized by comprising: a semiconductor chip; a waveguide provided in the semiconductor chip; a traveling wave electrode having an input part and an output part for modulating light passing through the waveguide; a power supply line , connected to the input part via a first wire; a terminal resistance, connected to the output part via a second wire, wherein the capacitance between the output part and the ground point is larger than the capacitance between the input part and the ground point big.

According to the present invention, the modulation frequency band can be sufficiently expanded.

Description of drawings

FIG. 1 is a plan view showing an optical modulator according to Embodiment 1. As shown in FIG.

Fig. 2 is a sectional view taken along line A-A' of Fig. 1 .

FIG. 3 is a plan view showing an optical modulator of Comparative Example 1. FIG.

FIG. 4 is a graph showing actual measurement values of transmission characteristics S21 and reflection characteristics S11 of the optical modulator of Comparative Example 1. FIG.

5 is a graph showing actual measurement values of transmission characteristics S21 and reflection characteristics S11 of the optical modulator of Comparative Example 1. FIG.

FIG. 6 is a plan view showing an optical modulator of Comparative Example 2. FIG.

FIG. 7 is a circuit diagram showing effective output impedance of Comparative Example 2. FIG.

FIG. 8 is a circuit diagram showing effective output impedance in Embodiment 1. FIG.

FIG. 9 is a graph showing actual measurement values of transmission characteristics S21 and reflection characteristics S11 of the optical modulator of Comparative Example 2. FIG.

10 is a graph showing actual measurement values of the transmission characteristic S21 and the reflection characteristic S11 of the optical modulator according to the first embodiment.

FIG. 11 is a plan view showing an optical modulator according to Embodiment 2. FIG.

FIG. 12 is a plan view showing an optical modulator according to Embodiment 3. FIG.

FIG. 13 is a plan view showing an optical modulator according to Embodiment 4. FIG.

FIG. 14 is a plan view showing an optical modulator according to Embodiment 5. FIG.

FIG. 15 is a plan view showing a modified example of the optical modulator of Embodiment 5. FIG.

FIG. 16 is a plan view showing an optical modulator according to Embodiment 6. FIG.

FIG. 17 is a plan view showing an optical modulator according to Embodiment 7. FIG.

FIG. 18 is a plan view showing an optical modulator according to Embodiment 8. FIG.

Explanation of reference signs:

10 semiconductor chip

12 waveguides

14 Traveling wave electrodes

14a Input section

14b output section

16 ground wire

18 first wire

20 power supply line

22 Second wire

26 Terminal resistance

50 The first short stub (stub)

52 first insulating film

54 Second insulating film

56 Second stub.

Detailed ways

An optical modulator according to an embodiment of the present invention will be described with reference to the drawings. The same reference signs are attached to the same or corresponding structural elements, and overlapping descriptions may be omitted.

Embodiment 1

FIG. 1 is a plan view showing an optical modulator according to Embodiment 1. As shown in FIG. This optical modulator is a traveling wave Mach-Zehnder optical modulator using a semiconductor as a material.

Inside the semiconductor chip 10, a waveguide 12 having interference arms 12a, 12b is arranged. A traveling wave electrode 14 is provided on the side of the interference arm 12 a on the semiconductor chip 10 . In the vicinity of the traveling wave electrode 14, a ground line 16 is provided at a distance from the ground. The traveling wave electrode 14 and the ground wire 16 are made of metal such as gold plating.

The power supply line 20 is connected to the input portion 14 a of the traveling wave electrode 14 via the first lead wire 18 . The terminal resistance 26 is connected to the output part 14 b of the traveling wave electrode 14 via the second lead wire 22 and the terminal line 24 . The impedance of the power supply line 20 is 50Ω, and the resistance value of the terminal resistor 26 is 25Ω. The traveling-wave electrode 14 generates a traveling-wave electric field based on the electric signal input from the power supply line 20 , and modulates light passing through the interference arm 12 a of the waveguide 12 by the electric field.

Grounding lines 28 are provided on both sides of the power supply line 20 , and grounding lines 30 are provided on both sides of the terminal line 24 . The ground wire 28 is connected to the input side of the ground wire 16 via a wire 32 and the ground wire 30 is connected to the output side of the ground wire 16 via a wire 34 .

The area of the bonding pad of the output part 14b is larger than the area of the bonding pad of the input part 14a. Therefore, the capacitance between the output part 14b and the ground point is larger than the capacitance between the input part 14a and the ground point. Furthermore, a traveling wave electrode, a ground line, a power supply line, a terminal resistor, and the like are similarly provided on the side of the interference arm 12b.

Fig. 2 is a sectional view taken along line A-A' of Fig. 1 . On the n-type InP substrate 36 , a core layer 38 , a p-type InP layer 40 , and a contact layer 42 of InGaAs, InP, or the like are laminated in this order. Halfway from the contact layer 42 to the n-type InP substrate 36 is etched to form a high-mesa waveguide.

The traveling wave electrode 14 is connected to the contact layer 42 on the ridge. The ground line 16 is provided on the n-type InP substrate 36 on both sides of the traveling wave electrode 14 . The back electrode 44 is ohmically connected to the back surface of the n-type InP substrate 36 .

The core layer 38 is an undoped multiple quantum well, and is made of a material having a higher refractive index than the p-type InP layer 40 such as InGaAsP or AlGaInAs. The thickness of the core layer 38 is 0.1 μm˜0.6 μm, and the width of the high mesa waveguide is 1 μm˜3 μm.

Next, the operation of the optical modulator of this embodiment will be described. Incident light 46 such as laser light enters the waveguide 12, is split into two beams, and propagates through the two interference arms 12a and 12b of the waveguide 12, respectively. Then, the two lights are combined into one light, which is emitted as output light 48 .

When voltages of different magnitudes are applied to the two interference arms 12a and 12b, the refractive indices of both have different values from each other. When the difference in the refractive index is Δn, the length of the portion where the voltage is applied to the two interference arms 12a and 12b is L, and the wavelength of the light propagating through the interference arms is λ, the two interference arms 12a , 12b of the phase of the light produced a difference Δφ.

When the phase difference Δφ of the light is nπ (n is 0 or an even number), the light propagating through the two interference arms 12 a and 12 b is combined and strengthened. On the other hand, when the phase difference Δφ is kπ (k is an odd number), the lights propagating through the two interference arms 12 a and 12 b are combined and canceled out. Thus, the intensity of the light can be modulated by means of the voltage applied to the two interference arms 12a, 12b. Furthermore, when a modulation voltage is applied so that the phase difference Δφ reciprocates between the state of nπ and the state of (n+2)π, the phase of light can be modulated.

Next, the effect of this embodiment will be described in comparison with a comparative example. FIG. 3 is a plan view showing an optical modulator of Comparative Example 1. FIG. In Comparative Example 1, unlike Embodiment 1, the input unit 14 a and the output unit 14 b have the same area. Therefore, the capacitance between the output part 14b and the ground point is the same as the capacitance between the input part 14a and the ground point.

4 and 5 are diagrams showing actual measurement values of the transmission characteristic S21 and the reflection characteristic S11 of the optical modulator of Comparative Example 1. FIG. In FIG. 4, the terminal resistance is 50Ω, and the input and output impedances are symmetrical. In FIG. 5, the terminal resistance is 25Ω. The width of the high-mesa waveguide was 1.8 μm, the thickness of the core layer was 0.35 μm, and the impedance of the waveguide 12 (the impedance of the traveling-wave electrode 14 ) was about 35Ω.

Here, the wider the transmission band of electricity is, the wider the modulation band of light modulated by electricity becomes. In general, the modulation band is maximized when the group velocity of microwaves propagating in the waveguide coincides with the propagation velocity of light. In particular, when the width of the interference arm of the Mach-Zehnder optical modulator is 2 mm or less, the group velocity of microwaves and the propagation velocity of light do not depend much, and the transmission frequency band of electricity and the modulation frequency band of light substantially coincide.

As can be seen from FIGS. 4 and 5 , when the terminal resistance is 50Ω, the frequency band fc is 13 GHz, and when the terminal resistance is 25Ω, the frequency band fc is 21 GHz. Here, when the input/output impedance is 50Ω, the waveguide 12 having a relatively low impedance of 35Ω becomes a capacitance, and the frequency band is limited. On the other hand, when the input impedance is 50Ω and the output impedance is 25Ω, the impedance of the waveguide 12 of 35Ω is approximately an intermediate value between them. Therefore, it is possible to expand the modulation frequency band by operating with impedance matching.

In addition, if the input and output impedance is set to 35Ω, it will completely match the impedance of the waveguide 12 . However, it is difficult to make the input impedance 35Ω due to the modulation driver or the power supply circuit. In addition, when the impedance (input impedance) of the power supply line 20 is set to 35Ω, the electric signal feels the inductance of the first lead wire 18 very strongly, and the electrical reflection in the power supply line 20 increases. Therefore, it is necessary to set the impedance of the power supply line 20 to a value close to 50Ω.

As described above, in order to expand the modulation frequency band, the impedance is set so as to satisfy the formula (2).

Here, Zin is the impedance (input impedance) of the power supply line 20 , Zwg is the impedance of the waveguide 12 , and Ro is the resistance value of the terminal resistor 26 .

And, it is further preferable to set the impedance so as to satisfy the formula (3).

However, due to the inductance L of the second lead wire 22 , the impedance jLω of the second lead wire 22 for a high frequency (for example, 20 GHz) rises and becomes higher than the resistance value of the terminating resistor 26 . Therefore, in Comparative Example 1, even if the resistance value Ro of the terminating resistor 26 is reduced, since the impedance jLω of the second wire 22 is added to the output impedance, the electrical reflection of the output part increases, the transmission characteristic deteriorates, and the modulation The frequency band becomes smaller.

FIG. 6 is a plan view showing an optical modulator of Comparative Example 2. FIG. In Comparative Example 2, the terminal line 24 is enlarged so as to function as a stub. FIG. 7 is a circuit diagram showing effective output impedance of Comparative Example 2. FIG. In Comparative Example 2, the capacitance C of the stub is connected in series with the second wire 22 . Therefore, when the impedance jLω of the second wire 22 becomes large (for example, greater than 25Ω), no matter how much the capacitance C of the stub is increased, the effective output impedance Zout cannot be reduced (for example, less than 25Ω).

FIG. 8 is a circuit diagram showing effective output impedance in Embodiment 1. FIG. In this embodiment, the capacitor C is connected in parallel with the second wire 22 . Therefore, when the capacitance C is increased, its impedance (1/jCω) decreases, offsetting the increase of the impedance jLω (for example, above 25Ω) of the second wire 22, and the effective output impedance Zout can be reduced (for example, below 25Ω) . As a result, since the reflection and transmission of electricity in the output unit 14b can be improved, the modulation frequency band can be sufficiently expanded.

FIG. 9 is a graph showing actual measurement values of transmission characteristics S21 and reflection characteristics S11 of the optical modulator of Comparative Example 2. FIG. 10 is a graph showing actual measurement values of the transmission characteristic S21 and the reflection characteristic S11 of the optical modulator according to the first embodiment. Obviously, the modulation frequency band of Embodiment 1 is wider than that of Comparative Example 2.

In addition, the smaller the impedance of the output portion 14b of the traveling wave electrode 14 is, the larger the difference from the impedance jLω of the second lead wire 22 is, and it is easily affected by it. Therefore, when formula (4) is satisfied, the above-mentioned effect strongly appears,

Z1>Z2 and Zwg>Z2 (4)

Here, Z1 is the impedance of the input part 14 a of the traveling wave electrode 14 , and Z2 is the impedance of the output part 14 b of the traveling wave electrode 14 .

Embodiment 2

FIG. 11 is a plan view showing an optical modulator according to Embodiment 2. FIG. The width of the output portion 14b of the traveling wave electrode 14 greatly increases toward the output end, and the area of the output portion 14b becomes larger than the area of the input portion 14a. Therefore, the capacitance between the output part 14b and the ground point is larger than the capacitance between the input part 14a and the ground point. Therefore, similarly to Embodiment 1, the modulation frequency band can be sufficiently expanded.

Embodiment 3

FIG. 12 is a plan view showing an optical modulator according to Embodiment 3. FIG. A first stub 50 is connected to the output portion 14b. Therefore, the capacitance between the output part 14b and the ground point is larger than the capacitance between the input part 14a and the ground point. Therefore, similarly to Embodiment 1, the modulation frequency band can be sufficiently expanded.

Embodiment 4

FIG. 13 is a plan view showing an optical modulator according to Embodiment 4. FIG. The traveling wave electrode 14 is not a GSG electrode as in the first embodiment. The area of the pad of the output unit 14b is three times the area of the pad of the input unit 14a. Therefore, the capacitance between the output part 14b and the ground point is about 0.2 pF larger than the capacitance between the input part 14a and the ground point. Therefore, similarly to Embodiment 1, the modulation frequency band can be sufficiently expanded. In addition, the output portion 14b may be connected to the pn junction of the semiconductor chip 10 to increase the capacitance.

Embodiment 5

FIG. 14 is a plan view showing an optical modulator according to Embodiment 5. FIG. The width of the traveling wave electrode 14 becomes wider from the input side toward the output side. Thus, the transmission path capacitance becomes larger as it gets closer to the output side. Thereby, similarly to Embodiment 1, it is possible to sufficiently expand the modulation frequency band.

FIG. 15 is a plan view showing a modified example of the optical modulator of Embodiment 5. FIG. In this way, the width of the waveguide 12 may be made wider from the input side toward the output side in accordance with the change in width of the traveling wave electrode 14 .

Embodiment 6

FIG. 16 is a plan view showing an optical modulator according to Embodiment 6. FIG. First and second insulating films 52 and 54 are provided on the semiconductor chip 10 . The input part 14 a of the traveling wave electrode 14 is provided on the first insulating film 52 , and the output part 14 b is provided on the second insulating film 54 . Furthermore, the second insulating film 54 is thinner than the first insulating film 52 by about one-third. Therefore, the capacitance between the output part 14b and the ground point is about 0.2 pF larger than the capacitance between the input part 14a and the ground point. Thereby, similarly to Embodiment 1, it is possible to sufficiently expand the modulation frequency band.

Embodiment 7

FIG. 17 is a plan view showing an optical modulator according to Embodiment 7. FIG. The distance between the output part 14b and the ground line 16 is smaller than the distance between the input part 14a and the ground line 16 . Accordingly, the capacitance between the output unit 14b and the ground is larger than the capacitance between the input unit 14a and the ground. Therefore, similarly to Embodiment 1, the modulation frequency band can be sufficiently expanded.

Embodiment 8

FIG. 18 is a plan view showing an optical modulator according to Embodiment 8. FIG. Since the impedance of the waveguide 12 is small, electric reflection occurs at the first wire 18 connecting the traveling wave electrode 14 and the power supply line 20 . Therefore, in this embodiment, the second stub line 56 connected to the end of the power supply line 20 is provided. The second stub 56 acts as a capacitor to offset the inductance component of the first wire 18 . Thereby, electrical reflection at the first wire 18 is reduced.

Furthermore, in Embodiments 1 to 8 described above, a case where the present invention is applied to a traveling-wave Mach-Zehnder optical modulator will be described. Not limited thereto, the present invention can also be applied to a traveling wave type electric field absorption modulator, and the same effect can be obtained.

Claims (7)

1. An optical modulator, characterized in that it has: semiconductor chips; a waveguide disposed within the semiconductor chip; a traveling wave electrode having an input and an output for modulating light passing through the waveguide; a power supply line connected to the input part via a first wire; and a terminating resistor, connected to the output via a second wire, The capacitance between the output part and the ground point is larger than the capacitance between the input part and the ground point. 2. The light modulator of claim 1, wherein The output portion has a larger area than the input portion. 3. The light modulator of claim 1, wherein There is also a first stub connected to the output. 4. The light modulator of claim 1, wherein further comprising first and second insulating films provided on the semiconductor chip, the input portion is disposed on the first insulating film, the output portion is provided on the second insulating film, The second insulating film is thinner than the first insulating film. 5. The light modulator of claim 1, wherein further comprising a grounded ground line provided on the semiconductor chip at a distance from the vicinity of the traveling wave electrode, The distance between the output part and the ground line is smaller than the distance between the input part and the ground line. 6. The optical modulator according to any one of claims 1 to 5, wherein: The resistance value of the terminal resistor is smaller than the impedance of the power supply line. 7. The optical modulator according to any one of claims 1 to 5, wherein: There is also a second stub connected to the power supply line.

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